Introduction to CNC cutting

v0.6 by Ger Walsh - August 2023

Intro Image

This is a guide to the various steps of using the ShopBot CNC Router. If you have not received the Induction on the machine, including the Health and Safety presentation for the machine, you may not use this machine. Please refer to our workshop safety information in regard to using the CNC router.

The Shopbot is a factory-built CNC Router able to cut wood, plastic, foam, composites and some soft metals. Maximum material size: 2440 x 1220 x 150mm. Many different sizes and types of cutting bits can be used.

BlackFoot CNC

Blackfoot CNC, built in SAUL, Limerick 2011

Steps

  1. Prepare your Design with a CAD program - AutoCAD, Solidworks or similar. Depending on your CAM program, gerber and 3D files (like .STL, .3DM or .STEP) might work here too.

  2. Use a CAM program to prepare your job, eg. V-Carve Pro, AlphaCAM or similar

  3. Set up the ShopBot Router and cutting your Part File

CAD (short for Computer Aided Design)

Here we are mainly talking about 2D vector drawings. Two common filetypes are .DWG and .DXF which can be prepared in programmes like AutoCAD, SolidWorks, Fusion360, FreeCAD, DraftSight etc. to name but a few examples... It is also possible to do a certain amount of editing on some of the CAM programmes, but it is probably preferable to have your drawing as ready as possible in your native drawing program. It is possible to export your designs from Inkscape and Illustrator too.

To prepare drawings for use with the ShopBot, we use a software called V-Carve Pro, a software made by Vectric for use with the ShopBot, and many other CNC Machines.

V-Carve can open CAD files in .DWG and .DXF formats, bitmap images (.JPG, etc.), and 3D .STL files. A number of packages, Fusion360 from Autodesk and SolidWorks from Dessault Systems, have included in-built CAM functionality for a number of years.

CAM (Computer Aided Manufacturing)

CAM softwares take geometries from your drawings and generate ToolPaths for your G-Code file. Generally, most 2D cuts can be achieved with one of the following ToolPaths:

  • Profile (linear profile, with options to automatically offset the ToolPath inside or outside, based on the bit size)
  • Pocket (produces ToolPaths to remove the interior of a closed shape, to a depth specified)
  • Drill Hole Pattern (a pattern of points/centre points to lay out multiple drill holes)
  • V Carve Profile,

3d Axes, with overlays

The CAM software selects and applies these profiles and their attributes in a new file. Each set is called a Toolpath. Toolpaths contain information like feed rate (horizontal speed, ips or mm/s), spindle rate (rpm), pass/cutting depth(mm), and the coordinates for the tool to follow. ToolPaths are then exported and sent to the machine using SB3 (on another machine, it might be Mach3, or UGS, or similar). The file extension may be .sbp or .rc, but it is commonly called a GCode File.

Bits

Some of the typical bits used for CNC routing:

Various Bits

image Luis Rodriguez, Make Magazine

Choice is based first on material(engineered boards, solid/hard timber, soft/hard plastic, metal), then on application (are you cutting through? Are you carving letters or doing 3D profiles?).

For example, a good choice for timber and engineered boards, generally, would be to use

  • a Compression Bit for outside profiles(the 6th tool in the image above),
  • a Downcut Bit for pockets (number 3 above),
  • and a V-bit for V-carving letters(number 5 above).

You can use roundnose or roundover bits for flute-shaped channels and fillets. V-bits also make good chamfers, and are available for various angles. Small-tapered roundnose bits are good for 3D finishing work.

V-Carve Pro Tool Library, Main Screen

Programs like V-Carve Pro, store commonly reused values, like feed-rate, pass depth and so on in their Tool Database. You can find settings from the tool manufacturers themselves, like cutter-shop.com, Onsrud. Use of a Tool Database makes it easier to get started. Typically setting are given in ranges that you can adjust as you get more familiar with them.

There is a download link to our tool database on the V-Carve Pro page.

Luis Rodreguez has a good article on this, if you want more information: article.

Tool parameters

  • Tool Type can be set to "Ball Nose", "End mill", "V-Bit", "Engraving", or a number of others. This determines other properties that can be entered under "Geometry".

  • Diameter is Diameter. (Angle, Taper, and other options will appear here for other types of tool).

  • Pass Depth is the maximum allowable depth that the tool can cut into the material. A good rule of thumb is the same as the diameter. If you are cutting 15mm material with a Pass Depth of 6.35mm, your first depth will be 6.35; then 12.7, then 15mm, and so your toolpath will contain three "passes".

  • Stepover is how much overlap there is if trying to cut out a shape. Can be specified by distance or %. Relevant for Pocket profiles or 3D Roughing paths.

  • Spindle Speed is taken in revolutions per minute, rpm.

  • Feed Rate is how fast the tool is allowed to move horizontally. If changing to other units, note that it does not recalculate the other Feeds and Speeds for you.

  • Plunge Rate is how slow or fast it moves vertically in material.

To see more about Tooling & Tool Databases see: this video from Shoppbot Tools Ltd (external link to YouTube.com).

Toolpath parameters

  • Start Depth / Cut Depth.

  • Tool - chosen preset from tool database.

  • Particular options related to the type of toolpath:

    • Alignment (Inside/Outside/On),
    • Direction (Climb or Conventional), and Ramping,
    • Clearance or Roughing/Finishing Strategy (Offset, Raster, 3D Raster along Y direction),
    • Clearance Tool (A second, bigger, tool to help clear large areas more efficiently),
    • Pecking and Dwelling (to do with Drilling paths),
    • Boundary Limit (Clips down 3D layers to avoid problems at the edge).
  • Tabs. Tabs are breaks in profiles or 3D work so that nothing moves around while you work.

Other

Material Dimensions, Safe Height, Origin/Offset are set on a per-file basis. Set these when you open a new file, or by clicking this icon, Job Settings, on your tool pallet (top left).

Chipload (optional)

Profile Chipload example

Often the settings you find online, will give you a range of values, or else they will give you something called "Chipload". You can think of Chipload as the amount of material removed by each turn or swipe of the cutter. There is a formula, which will help you figure out the above values based on the Chipload given.

Chipload = Feed Rate  / (RPM x no. of flutes)

...where "Feed Rate" is in inches per minute and "RPM" is revolutions per minute. Some CAM programs will have a calculator for this built into the software. There are some softwares online that can help, but as long as you use the correct units, you can use this formula yourself.

Online Resources

50-digital-wood-joints

50 Digital Wood Joints: an open source collection of designs for CNC cut timber joints (license: CC BY-NC-SA 3.0 Jochen Gros)




opendesk

opendesk.cc is an online network for furniture users, designers and makers - it's a great online digital repository for cnc-cut furniture designs, to download for free.




WikiFactory

Wikifactory.com is a product development platform, and also provides the platform for projects.fablabs.io.




sketchchair-pic

sketchchair-anim

sketchchair.cc is an open source project, supported and developed by a number of contributors. SketchChair is designed to give anybody the opportunity to design a unique one-off original chair that is personal to them. Any of the chairs featured on the site are freely available to be downloaded and edited by anybody, allowing chair designs to evolve as users continually refine and modify them.




akerkits

AKERKits are another resource for downloading great designs, this time with a focus on urban gardening, growing units, and beehives.




CNC Maker Bench Maker Bench Made with a CNC Router is an article from Make magazine. It shows the whole process from design (using custom parametric software), material selection, cutting.

More Examples:

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